support interactive iframe backgrounds in vertical stacks

css/reveal.css

@@ -531,7 +531,8 @@ body {
 
 .reveal .slides > section.stack {
   padding-top: 0;
-  padding-bottom: 0; }
+  padding-bottom: 0;
+  pointer-events: none; }
 
 .reveal .slides > section.present,
 .reveal .slides > section > section.present {

css/reveal.scss

@@ -640,6 +640,7 @@ $controlsArrowAngleActive: 36deg;
 .reveal .slides>section.stack {
 	padding-top: 0;
 	padding-bottom: 0;
+	pointer-events: none;
 }
 
 .reveal .slides>section.present,